This invention relates to surface modifications to waveguides to improve power transmission, and more particularly to an improved method of applying a highly adherent carbon coating to the waveguide interior. The waveguide is a standard means for transmitting electromagnetic energy. In the development of a nuclear fusion reactor, waveguides are used for transmitting radiofrequency (RF) energy to the plasma for heating and for driving current. A very large amount of power must be transmitted to the plasma before sustained nuclear fusion can occur. The waveguides used on the PLT (Princeton Large Torus) for lower hybird heating and current drive experiments are made of stainless steel and are subject to high power (greater than 80 KW) RF breakdown. The waveguides used for ion cyclotron resonance frequency (ICRF) experiments are made of copper and experience similar breakdown. Factors contributing to this breakdown are gas evolved from waveguide walls, electron multipaction, photoelectron emission and arcing. In addition to eliminating or suppressing these breakdown factors, a coating suitable for use on a treated waveguide for transmitting power to a plasma must also have a low Z (atomic number) and be operable near the high magnetic fields confining the plasma.
Due to its low Z, adsorbed gas free surface, low secondary electron yield, carbon is a desirable coating material, but prior are methods of producing carbon coatings have not proven satisfactory. Deposition of soot from acetylene and propane rich flames produces non-uniform thickness and poor adhesion. Coatings produced by pyrolysis of commercial carbon rich paints have uniform thickness but poor adhesion. Coatings produced by heating a film formed by electrodepositing carbon suspended in a water soluble resin are too thick, resulting in power attenuation in a waveguide. Furthermore, methods that use water based products result in oxide formations which reduce conductivity.
Therefore, it is an object of the present invention to provide a method of treating the interior surfaces of a waveguide to improve power transmission.
It is another object of the present invention to provide a method of producing a thin, durable carbon coating with good adherence and good uniformity.
It is yet another object of the present invention to treat waveguide surfaces to improve resistance to to high power RF breakdown.
Additional objects, advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practicing the invention.